This application claims priority to Austrian application No. A 940/99, filed on May 27, 1999, herein incorporated by reference.
Aqueous formulations containing polyvinylbutyral (PVB) are known. The majority of such systems are based on the use of ionic emulsifiers (salts of fatty acids, or alkanesulfonates or alkyllaurylsulfonates: U.S. Pat. Nos. 2,686,724, 2,532,223, 2,611,755, 2,509,667, DE-A 27 52 054) or nonionic emulsifier types such as polyvinyl alcohol (JP-A 58/026374), ethoxylated fatty alcohols or reaction products of polyalkylene glycols with polyepoxides (EP-A 0 638 606).
All of these externally emulsified systems, however, have the general disadvantage that the coating materials cannot be pigmented directly owing to the inadequate shear stability of the emulsions. Added to this are the disadvantages occasioned by the type of emulsifier. In the case of high molar mass polyethylene glycol types, there is a rapid decrease in the water resistance of the films and, with the use of low molar mass ionic emulsifiers, it is possible for the emulsifier molecules to migrate in the film and to be leached from the resin system, so causing a continually changing surface composition and changing film properties.
Self-emulsifying aqueous polyvinylbutyral dispersions have also been described: the groups responsible for dilutability in water (which are ionic or nonionic in nature) are introduced into the polymer before the acetalization process.
For the preparation of the polyvinyl alcohol, DE-A 32 46 605 uses a polyethylene grafting base which is subsequently acetalized and which owing to the presence of polyethylene glycol segments leads to water-dilutable end products.
In DE-A 42 35 151, a polyvinyl alcohol copolymer comprising ethylenesulfonate (in salt form) is prepared which is subjected to acetalization and which owing to the presence of sulfonate groups gives water-dilutable products.
Since aqueous polyvinyl alcohol solutions are the starting point in both cases, the resulting dispersions are obtained with a very low solids content. Subsequent blending with plasticizing resins, which are very commonly used to improve film formation, is virtually impossible owing to the reduced compatibility of the modified products. Furthermore, owing to the polyethylene glycol structure or, respectively, to the sulfonate groups, the products are highly sensitive to water and do not satisfy the requirements for industrial use.
Progress was achieved by virtue of the anionically stabilized dispersions described in EP-A 0 741 155; however, their corrosion protection effect is inadequate, since the anionic stabilization by the addition of phosphoric acid, which is usual in conventional anticorrosion primers, is impossible owing to the coagulation of the dispersions which it triggers.
The German Offenlegungsschrift DE-A 196 18 379 describes cationically stabilized epoxy resin dispersions for cataphoretic electrodeposition coating. These dispersions comprise, as a second resin component, a vinyl acetate copolymer, which may also have vinylacetal units.
An object of this invention was therefore to prepare self-emulsifying PVB dispersions which permit addition of phosphoric acid and therefore have the same corrosion protection properties as conventional flash primers.
It has now been found that by modifying the PVB with tertiary alkanolamines (alone and in combination with cationically stabilized resins) it is possible to obtain water-dilutable primers which satisfy the technical requirements. It is also possible in accordance with the invention in the same way to convert the optimum resin composition for different application purposes (low-, normal- and high-acetalized PVB types, different degrees of polymerization, or combinations with plasticizing resins) into aqueous dispersion form.
The invention accordingly provides water-dilutable PVB dispersions obtainable by a two-stage reaction, the first stage of which
at least one tertiary alkanolamine A and
at least one diisocyanate B are reacted to the theoretically calculated NCO content
and the amounts of components A and B are chosen such that the amount of substance of the diisocyanates B is from about 0.8 to about 1.3 times the amount of substance of the reactive hydroxyl groups in A
and thereafter in the second stage
a polyvinylbutyral C, dissolved in a solvent which is inert towards isocyanates, is reacted with the adduct AB from the first step until all of the isocyanate groups have undergone complete reaction, the number of hydroxyl groups in C being at least ten times the number of isocyanate groups in the reaction product AB,
and combining this amine-modified PVB solution, if desired, with
one or more compatible plasticizing resins D which is or are likewise water-dilutable by way of tertiary amino groups,
the resin mixture containing mass fractions of solids of
i) from about 20 to about 100% of modified polyvinylbutyral ABC and
ii) from 0 to about 80% of the suitable plasticizing resin D or of a mixture of two or more such suitable plasticizing resins
with the proviso that the sum of i) +ii) is 100% and the resulting resin mixture has tertiary amino groups corresponding to an amine number of from about 20 to about 80 mg/g of resin solids.
In the first stage it is essential that the amount of substance of the diisocyanates B is from about 0.8 to about 1.3 times the amount of substance of the reactive hydroxyl groups in A, meaning that the ratio of the amount of substance of the isocyanate groups n(NCO) in component B to the amount of substance of the hydroxyl groups n(OH) in component A is
n(NCO)/n(OH)=from about 1.6 to about 2.6.
On average, therefore, in each diisocyanate molecule used roughly more than one (to be precise, from 0.8 to 1.3) unreacted isocyanate group remains, the other isocyanate groups having been transformed into urethane groups by reaction with the alkanolamine. It is advantageous to use a slight excess of isocyanate groups in order to compensate for their reaction with any possible impurities. Where monoalkyldialkanolamines are used additionally or exclusively as component A, an excess of isocyanate groups is unnecessary; and in fact, some (up to about 50%, preferably up to 30%) of the diisocyanates B can be replaced by monoisocyanates. The amine number (AN) is defined in accordance with DIN 53 176 as the ratio between the mass mKOH of potassium hydroxide which consumes exactly the same amount of acid for neutralization as the sample under analysis and the mass mB of this sample (mass of the solid in the sample in the case of solutions or dispersions); its customary unit is xe2x80x9cmg/gxe2x80x9d.
Following neutralization of at least about 50%, preferably more than 70%, of the tertiary amino groups with (in)organic acids, the products of the invention are diluted with deionized water.
Depending on the viscosity of the product, the auxiliary solvent used is distilled off from the dispersion either before neutralization in vacuo or in an azeotrope (under reduced pressure as well if desired).
As component A, use is made of tertiary alkanolamines having one to two OH groups. Preference is given to dialkyl-monoalkanolamines, in which the alkyl radicals can be identical or different and are selected preferably from linear or branched alkyl radicals having 1 to 6 carbon atoms. The parent alkanolamine has 2 to 12 carbon atoms and carries the hydroxyl group and the amino group preferably in positions xcex1 and xcfx89. It is also possible to use dialkanol-alkylamines having 1 to 6 carbon atoms in the alkyl group and 2 to 12 carbon atoms in the alkanol group, the hydroxyl group likewise being positioned, preferably, to the xcex1xcfx89 ( amino group. In this case it is also possible to use those alkanol residues which carry more than one hydroxyl group (on different carbon atoms). Examples of such compounds are N-methyldiethanolamine, N,N-dimethylethanolamine or N,N-diethylethanolamine or diethylaminopropanediol. One particularly preferred group are the N,N-dialkylethanolamines.
In order to make it easier to conduct the synthesis, component A can be dissolved in an inert high-boiling solvent which is compatible with water (e.g., N-methylpyrrolidone) and which subsequently also permits improved leveling and improved film formation. Such coalescence agents are commonly used in mass fractions of from about 1 to about 20% (based on the final mass of solid resin). Solvents containing NCO-reactive groups (butyl glycol, glycol ethers, for example) are added, if appropriate, only after the isocyanate reaction has been concluded.
The diisocyanate component B comprises any desired organic diisocyanates having free (i.e., not blocked) isocyanate groups attached to aliphatic, cycloaliphatic, araliphatic and/or aromatic moieties. When aliphatic diisocyanates are used (in which case hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, isophorone diisocyanate and 1,4-diisocyanatocyclohexane are preferred), relatively elastic coatings are obtained; the use of aromatic diisocyanates (tolylene diisocyanate, diphenylmethane diisocyanate and naphthylene diisocyanate are preferred) leads to hard coatings. The selection is made depending on the viscosity of the resulting products and on the profile of requirements for the respective end use. As already stated above, a fraction of the diisocyanates can be replaced by monoisocyanates if alkyldialkanolamines are used as component A.
The reaction of component A with component B is strongly exothermic.
As component C it is possible to use commercially available PVB grades, the properties of the coating being decisively codetermined by the degree of polymerization and the degree of acetalization of the corresponding PVB grade. Examples of customary characteristics are as follows: viscosity (6% strength solution in methanol, 20xc2x0 C.) from 1 to 200 mPaxc2x7s; degree of acetalization from 65 to 90%, preferably from 70 to 85% (calculated as mass fraction of polyvinylacetal); acetate content from 2 to 5%, preferably from 3 to 4% (calculated as mass fraction of polyvinyl acetate); OH or vinyl alcohol group content from 10 to 30%, preferably from 11 to 20% (calculated as mass fraction of polyvinyl alcohol).
Plasticizing resins (component D) used are those resins which are compatible with polyvinylbutyral; that is, do not lead to macroscopic separation. They comprise primarily amino resins, phenolic resins, alkyd resins, and also low-epoxide or epoxide-free resins based on glycidyl ethers of dihydroxy aromatic compounds (especially the so-called phenoxy resins). Particularly suitable, for example, are reaction products of bisphenol A-based liquid resins ((copyright)Beckopox EP 140, specific epoxide group content about 5560 mmol/kg, xe2x80x9cepoxide equivalent weightxe2x80x9d EEW=180 g/mol) with (substituted) monophenols and bisphenols, which are dilutable in water by virtue of the incorporation of dialkylaminoalkylamines (e.g., diethylaminopropylamine) corresponding to an amine number of from 20 to 80 mg/g.
Within the coating formulation, these plasticizing resins may also serve as a vehicle for the optionally added phosphoric acid, which acts as a crosslinker. The phosphoric acid is incorporated preferably in combination with a thickener and together with the plasticizing resin into the PVB dispersion to which the customary coatings additives have been added; alternatively, it can be added after the PVB dispersion and the plasticizing resin have already been mixed. The mass fraction of phosphoric acid in the finished coating material is preferably from about 0.1 to 5%.
Films on the basis of the dispersions of the invention exhibit excellent adhesion to metal, plastics and glass. Primer systems produced with them also exhibit good intercoat adhesion, so that the overcoatability is good as well. Depending on the nature of the plasticizing resin, it is also possible to produce films having excellent hardness, water resistance and corrosion protection effect.
Dispersions of the invention can therefore be used as binders for wash primers or xe2x80x9cflash primersxe2x80x9d, industrial coatings, heat-resistant coating materials (e.g., radiator coatings), packaging coatings, adhesion primers, welding primers, wood flooring varnishes, traffic paints, and adhesives.